John Gilmore writes:
> > > > Are there network devices that are neither point-to-point, nor do they
> > > > have broadcast support?
>
> It seems to me that cellular radio systems are neither point-to-point,
> nor do they support broadcast. And they keep moving further and further
> toward being IP-based (e.g. phone calls on your LTE phone use SIP over
> UDP over IP over cellular!). But given the proprietary nature of most
> cellphone radio chips and their firmware, it's possible that Linux has
> never built a fully capable network interface to talk to them. I don't
> think we should attempt to rule it out, though.
I found in include/uapi/linux/if_link.h the following classification
about this:
IFF_LOOPBACK, IFF_BROADCAST and IFF_POINTOPOINT are
more not changeable by user. They describe link media
characteristics and set by device driver.
Comments:
- Combination IFF_BROADCAST|IFF_POINTOPOINT is invalid
- If neither of these three flags are set;
the interface is NBMA.
https://en.wikipedia.org/wiki/Non-broadcast_multiple-access_network
The likeliest example there in terms of running IP directly over such a
network would presumably be ATM, which had a whole series of RFCs about
how to make this work, and which is supported by the Linux kernel (e.g.
net/atm/clip.c, for "Classical IP over ATM" following RFC 1577). That
includes, for example, a non-broadcast-based ARP mechanism!
I'll try to figure out what a Linux ATM interface does about IP broadcasts.
It seems like the RFC 1577 instruction is to still allow IP broadcasts
but accept that they won't be mapped onto link-layer broadcasts:
ATM does not support broadcast addressing, therefore there are no
mappings available from IP broadcast addresses to ATM broadcast
services. Note: this lack of mapping does not restrict members from
transmitting or receiving IP datagrams specifying any of the four
standard IP broadcast address forms as described in [8]. Members,
upon receiving an IP broadcast or IP subnet broadcast for their LIS,
MUST process the packet as if addressed to that station.
In other IP-over-ATM methods it appears that broadcasting is emulated by
a lower layer.
https://www.cse.wustl.edu/~jain/cis788-97/ftp/ip_over_atm/index.html#3
